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Molecular and Cellular Biology, August 2004, p. 6967-6979, Vol. 24, No. 16
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.16.6967-6979.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The In Vivo Activity of Ime1, the Key Transcriptional Activator of Meiosis-Specific Genes in Saccharomyces cerevisiae, Is Inhibited by the Cyclic AMP/Protein Kinase A Signal Pathway through the Glycogen Synthase Kinase 3-ß Homolog Rim11

Ifat Rubin-Bejerano ,^, Shira Sagee, Osnat Friedman, Lilach Pnueli, and Yona Kassir^*

Department of Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel

Received 19 January 2004/ Returned for modification 8 March 2004/ Accepted 20 May 2004

Phosphorylation is the main mode by which signals are transmitted to key regulators of developmental pathways. The glycogen synthase kinase 3 family plays pivotal roles in the development and well-being of all eukaryotic organisms. Similarly, the budding yeast homolog Rim11 is essential for the exit of diploid cells from the cell cycle and for entry into the meiotic developmental pathway. In this report we show that in vivo, in cells grown in a medium promoting vegetative growth with acetate as the sole carbon source (SA medium), Rim11 phosphorylates Ime1, the master transcriptional activator required for entry into the meiotic cycle and for the transcription of early meiosis-specific genes. We demonstrate that in the presence of glucose, the kinase activity of Rim11 is inhibited. This inhibition could be due to phosphorylation on Ser-5, Ser-8, and/or Ser-12 because in the rim11S5AS8AS12A mutant, Ime1 is incorrectly phosphorylated in the presence of glucose and cells undergo sporulation. We further show that this nutrient signal is transmitted to Rim11 and consequently to Ime1 by the cyclic AMP/protein kinase A signal transduction pathway. Ime1 is phosphorylated in SA medium on at least two residues, Tyr-359 and Ser-302 and/or Ser-306. Ser-302 and Ser-306 are part of a consensus site for the mammalian homolog of Rim11, glycogen synthase kinase 3-ß. Phosphorylation on Tyr-359 but not Ser-302 or Ser-306 is essential for the transcription of early meiosis-specific genes and sporulation. We show that Tyr-359 is phosphorylated by Rim11.

I.R.-B. and S.S. contributed equally to this article.

Present address: Whitehead Institute for Biomedical Research, Cambridge, Mass.

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